1 Constraint Satisfaction Problems Slides by Prof WELLING.

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Constraint Satisfaction Problems

Slides by Prof WELLING

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Constraint satisfaction problems (CSPs)

CSP: state is defined by variables Xi with values from domain Di

goal test is a set of constraints specifying allowable combinations of values for subsets of variables

Allows useful general-purpose algorithms with more power than standard search algorithms

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Example: Map-Coloring

Variables WA, NT, Q, NSW, V, SA, T

Domains Di = {red,green,blue}

Constraints: adjacent regions must have different colors e.g., WA ≠ NT

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Example: Map-Coloring

Solutions are complete and consistent assignments, e.g., WA = red, NT = green,Q = red,NSW = green,V = red,SA = blue,T = green

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Constraint graph

Binary CSP: each constraint relates two variables Constraint graph: nodes are variables, arcs are

constraints

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Varieties of CSPs

Discrete variables finite domains:

n variables, domain size d O(d n) complete assignments e.g., 3-SAT (NP-complete)

infinite domains: integers, strings, etc. e.g., job scheduling, variables are start/end days for each job: StartJob1 + 5 ≤ StartJob3

Continuous variables e.g., start/end times for Hubble Space Telescope

observations linear constraints solvable in polynomial time by linear

programming

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Varieties of constraints

Unary constraints involve a single variable, e.g., SA ≠ green

Binary constraints involve pairs of variables, e.g., SA ≠ WA

Higher-order constraints involve 3 or more variables, e.g., SA ≠ WA ≠ NT

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Example: Cryptarithmetic

Variables: F T U W R O X1 X2 X3

Domains: {0,1,2,3,4,5,6,7,8,9} {0,1} Constraints: Alldiff (F,T,U,W,R,O)

O + O = R + 10 · X1

X1 + W + W = U + 10 · X2

X2 + T + T = O + 10 · X3

X3 = F, T ≠ 0, F ≠ 0

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Real-world CSPs

Assignment problems e.g., who teaches what class

Timetabling problems e.g., which class is offered when and where?

Transportation scheduling Factory scheduling

Notice that many real-world problems involve real-valued variables

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Standard search formulation

Let’s try the standard search formulation.

We need:• Initial state: none of the variables has a value (color)• Successor state: one of the variables without a value will get some value.• Goal: all variables have a value and none of the constraints is violated.

N! x D^N

N layersWA NT TWA WA

WANT

WANT

WANT

NxD

[NxD]x[(N-1)xD]

NTWA

Equal!

There are N! x D^N nodes in the tree but only D^N distinct states??

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Backtracking (Depth-First) search

WAWA WA

WANT

WANT

D

D^2

• Special property of CSPs: They are commutative: This means: the order in which we assign variables does not matter.

• Better search tree: First order variables, then assign them values one-by-one.

WANT

NTWA

=

WANT

D^N

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Backtracking example

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Improving backtracking efficiency

General-purpose methods can give huge gains in speed: Which variable should be assigned next? In what order should its values be tried? Can we detect inevitable failure early?

We’ll discuss heuristics for all these questions in the following.

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Which variable should be assigned next? minimum remaining values heuristic

Most constrained variable:choose the variable with the fewest legal values

a.k.a. minimum remaining values (MRV) heuristic

Picks a variable which will cause failure as soon as possible, allowing the tree to be pruned.

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Which variable should be assigned next? degree heuristic

Tie-breaker among most constrained variables

Most constraining variable: choose the variable with the most

constraints on remaining variables (most edges in graph)

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In what order should its values be tried? least constraining value heuristic

Given a variable, choose the least constraining value: the one that rules out the fewest values in the

remaining variables

Leaves maximal flexibility for a solution. Combining these heuristics makes 1000

queens feasible

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Rationale for MRV, DH, LCV

In all cases we want to enter the most promising branch, but we also want to detect inevitable failure as soon as possible.

MRV+DH: the variable that is most likely to cause failure in a branch is assigned first. E.g X1-X2-X3, values is 0,1, neighbors cannot be the same.

LCV: tries to avoid failure by assigning values that leave maximal flexibility for the remaining variables.

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Can we detect inevitable failure early? forward checking

Idea: Keep track of remaining legal values for unassigned

variables that are connected to current variable. Terminate search when any variable has no legal values

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Forward checking


variables Terminate search when any variable has no legal values

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Forward checking



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Forward checking



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Constraint propagation

Forward checking only looks at variables connected to current value in constraint graph.

NT and SA cannot both be blue! Constraint propagation repeatedly enforces

constraints locally

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Arc consistency

Simplest form of propagation makes each arc consistent

X Y is consistent ifffor every value x of X there is some allowed y

constraint propagation propagates arc consistency on the graph.

consistent arc.

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Arc consistency

Simplest form of propagation makes each arc consistent

X Y is consistent ifffor every value x of X there is some allowed y

inconsistent arc.remove blue from source consistent arc.

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Arc consistency

Simplest form of propagation makes each arc consistent X Y is consistent iff

for every value x of X there is some allowed y

If X loses a value, neighbors of X need to be rechecked: i.e. incoming arcs can become inconsistent again (outgoing arcs will stay consistent).

this arc just became inconsistent

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Arc consistency

Simplest form of propagation makes each arc consistent X Y is consistent iff

for every value x of X there is some allowed y

If X loses a value, neighbors of X need to be rechecked Arc consistency detects failure earlier than forward

checking Can be run as a preprocessor or after each assignment

Time complexity: O(n2d3)

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Arc Consistency

This is a propagation algorithm. It’s like sending messages to neighbors on the graph! How do we schedule these messages?

Every time a domain changes, all incoming messages need to be re-send. Repeat until convergence no message will change any domains.

Since we only remove values from domains when they can never be part of a solution, an empty domain means no solution possible at all back out of that branch.

Forward checking is simply sending messages into a variable that just got its value assigned. First step of arc-consistency.

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Try it yourself

[R]

Use all heuristics including arc-propagation to solve this problem.

[R,B,G][R,B,G]

[R,B,G] [R,B,G]

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This removes any inconsistent values from Parent(Xj),it applies arc-consistency moving backwards.

BRG

BG

BRG

R G B

B G R R G B

Note: After the backward pass, there is guaranteed to be a legal choice for a child note for any of its leftover values.

a prioriconstrainednodes

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Junction Tree Decompositions

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Local search for CSPs Note: The path to the solution is unimportant, so we

can apply local search!

To apply to CSPs: allow states with unsatisfied constraints operators reassign variable values

Variable selection: randomly select any conflicted variable

Value selection by min-conflicts heuristic: choose value that violates the fewest constraints i.e., hill-climb with h(n) = total number of violated constraints

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Example: 4-Queens

States: 4 queens in 4 columns (44 = 256 states) Actions: move queen in column Goal test: no attacks Evaluation: h(n) = number of attacks

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Summary

CSPs are a special kind of problem: states defined by values of a fixed set of variables goal test defined by constraints on variable values

Backtracking = depth-first search with one variable assigned per node

Variable ordering and value selection heuristics help significantly

Forward checking prevents assignments that guarantee later failure

Constraint propagation (e.g., arc consistency) does additional work to constrain values and detect inconsistencies

Iterative min-conflicts is usually effective in practice

1 Constraint Satisfaction Problems Slides by Prof WELLING.

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